An overload monitoring is described in DE 10 2005 038 225.
Numerous types of power tools or electric garden tools may be operated without a connection to a power supply system. For storing the energy in these “cordless” power tools, garden tools or electric vehicles, electrochemical cells are used that are either permanently installed in the device or are situated in batteries removable from the device. In the following, the term “batteries” is used for all such cells, in particular, for rechargeable cells, e.g., Li-ion cells of a storage battery, as well.
In order to ensure the reliability and an energy storage capacity of the electrochemical cells that is stable in the long term, the cells should be protected from excessive heating due to extracting the energy stored in the cells too quickly.
This limiting may be implemented most easily by using components, which limit or interrupt the flow of current, either via direct heat transfer or amid too great a current flow and the resulting temperature increase of the component, for example, using conventional thermal fuses or bimetallic switches. A disadvantage of using such simple components, which are situated in the current path between the energy store and the load, is that in general, they may only constitute rough protection. This results from the fact that in order to operate power tools and similar devices, typically, relatively high currents must be drawn, whose order of magnitude is close to the overload or short-circuit currents whose flow is supposed to be prevented by the use of a fuse.
To limit the heating of the battery in operation, it is also conventional that closed-loop control or open-loop control, for instance, in the form of an electronic circuit, may be integrated into the device. In this context, the temperature of the cells is measured by one or more temperature sensors that are situated at or near the cell surface. Thus, for example, an NTC temperature sensor may be fixed to a location of the cell housing, from which one assumes that it accurately reflects the inside temperature of the cell. The temperature limits for the forced limitation of the load current are typically implemented as static quantities, and in the simplest case, as a maximum permissible temperature value. If exceedance of this temperature is detected at the cells by the temperature sensor, then the flow of current is interrupted. In this context, it is also conventional that several temperature limits may be implemented, at the exceedance of which the current may be increasingly limited in correspondingly many stages. A disadvantage of using static temperature limits is that, with regard to protecting the cells from overheating, these temperature limits only achieve the optimum effect for values of the discharging current defined very precisely. Thus, it is possible that the drawing of a very high current has caused the cell to already heat up very sharply in the interior, but that the inertia of the thermal conduction to the outer wall of the cell causes excessive heating of the cell to only be detected later and the necessary limiting of the current to only set in much later. This may mean that a critical situation is detected too late. In order to prevent this situation, the static temperature limits for limiting the maximum permissible discharging current may be set lower, so that intervention already takes place at an earlier stage of the heating of the cells. However, this means that in the event of more moderate discharges in which there is not such a sharp gradient between the interior temperature of the cell and the exterior temperature of the cell, the current is limited or interrupted earlier than actually intended. In order to counteract the impermissibly sharp temperature increase of the cells while drawing very high currents, the implementation of very low temperature limits may be necessary, which means that normal operation at moderate currents is already sharply limited or even becomes completely impossible at a slightly increased ambient temperature.
The above-mentioned publication, DE 10 2005 038 225, describes a method for detecting overload, in which, in order to limit the load current, the (measured) temperature of the cells is not used as a starting point, but in which the limitation of current is ascertained on the basis of the current drawn from the cells. In the above-mentioned, published patent application, it is proposed that thermal overload of components of the power train, e.g., the battery, be deduced from the difference between the value of the load current and at least one stored current value. It is also proposed that, for the subtraction, the simple, instantaneous value of the load current not be used, but a value weighted according to its magnitude, or a current value integrated over a particular time. Furthermore, it is proposed that in the event of a detected overload situation, the current be reduced or interrupted, using closed-loop control. It is mentioned that in the determination of system parameters, the operating temperature may enter into the arithmetic determination of an overload situation in the form of an input variable. A disadvantage of protecting the cells from overheating according to the conventional method, i.e., based only on the measurement of the current, is that possible changes in the system that may lead to an unexpectedly high temperature increase of individual components, for instance, the ageing of or damage to cells and the associated increase in internal resistance, are not taken into consideration. To be sure, the current is always measured, but the actual temperature at the cells is not measured or is only used as an input variable for the calculation. Seemingly disadvantageous is, in addition, the necessity of already having to set specific, maximum allowable currents during the design phase or, at the latest, during final implementation, which, in the event of a subsequent modification of the system, e.g., when using different cells (manufacturer, type), results in limitations, since a thermal characteristic of the utilized components that it always the same is necessary. Accordingly, an unauthorized exchange of the originally installed cells for cells having a different thermal characteristic or internal resistance may lead to safety problems.